There is a frequent need to convert variable-speed motive power produced by a prime mover, such as an aircraft jet engine, into constant-frequency AC power for one or more AC loads. Such a conversion can be effected by a variable-speed, constant-frequency (VSCF) power generating system which includes a brushless, synchronous generator coupled to the prime mover and a power converter coupled to the generator output windings which converts variable-frequency power produced by the generator into the constant-frequency power.
The power converter typically includes a rectifier which rectifies the variable-frequency power produced by the generator to produce DC power on a DC link and an inverter which converts the DC power into the constant-frequency AC power. The inverter may be of the stepped-waveform type in which a series of subinverters are coupled to a summing transformer that in turn produces a stepped AC waveform. Such an inverter produces an AC output having a harmonic content dependent upon the number of steps produced in each cycle of the output. Typically, the inverter includes either four or six 6-step inverters having outputs which are summed to produce 24-step or 36-step waveforms, respectively. Each 6-step subinverter includes six or a multiple of six power switches, and hence it can be seen that a relatively large number of power switches are utilized to produce the inverter output power. Such power switches or other components in the subinverters can occasionally fail, leading to an undesirable increase in harmonic content in the output or even rendering the entire inverter inoperative. In such a case, it is necessary to identify the failed switch or component by testing each until it is located and replacing same. This can be a time consuming process and can result in significant down time for the inverter.